1. Field of the Invention
The present invention relates to a control system for a positioning device. The present invention also relates to lithographic apparatus, a device manufacturing method, and a device manufactured thereby.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An examples of a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can include one or more programmable mirror arrays. More information on mirror arrays as here referred to can be found, for example, in U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193 and PCT Patent Application Publications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning devices as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be found, for example, in U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types to direct, shape or control the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796, incorporated herein by reference.
During manufacture it is necessary to move the substrate into the correct position for loading, unloading and exposure. It may also be necessary to move the mask. It is important that any movements are carried out as quickly and accurately as possible, to improve the throughput of the apparatus and the quality of the exposed substrates. Typically movement is controlled by a motion controller based on desired position, velocity and acceleration. The motion controller generates a desired current or voltage to be supplied to an actuator such that the desired position, velocity and acceleration are obtained. This desired current is input to an amplifier control system connected in a feedback loop to ensure the desired current or voltage is supplied to the actuator. A simplified diagram of the control system for a single phase actuator is shown in FIG. 5.
The motion controller provides the input of the desired current Is. The present value of the current Im is measured by a meter 12. The present value of the current Im is subtracted from the desired current Is by a subtractor 2 to give an error which is fed into the controller 4. The controller 4 calculates a new voltage setpoint, which is then converted by the amplifier 6 into a voltage which is provided to the actuator 10. The controller 4 is typically of the proportional, integral, differential (PID) type.
The parameters of the controller 4 are determined by considering the mechanical and electrical characteristics of the system. The response is constrained by the requirement to keep the system stable, accurate and within specified limits of phase difference. These constraints limit the speed at which movement to a desired position can be achieved.
The performance of the system has been improved by adding velocity and acceleration feedforward control into the calculation of the desired current Is in the controller 4. Such a system is illustrated in FIG. 6. A PID controller 24 has an input of the error between the measured position pm and a desired position ps from a setpoint generator. It outputs a force Fc to move into the desired position ps. A notch filter 26 suppresses a specific frequency in the closed loop system, to give better closed loop performance.
Calculators 32 and 28 respectively carry out an acceleration feedforward calculation and a velocity feedforward calculation using the desired values for acceleration as and velocity vs from the setpoint generator, and knowledge of the mechanical characteristics of the system. The results of the feedforward calculation, aforce-ff and vforce-ff, are added to the output of the notch filter 26 by summing devices 30, 34. A further notch is added by a notch filter 36. Finally the force is converted into a current setpoint Is by the converter 38 using the motor constant. The motor constant defines how many Newtons per amp the actuator delivers.
This modified system has improved response but suffers from the constraints of the current amplifier control system, which uses only feedback control. For example, if a current of 14A is desired it will not be produced instantaneously. The response will be delayed by the requirement to keep the current controller stable and any phase difference in the system. This places limitations on the speed of movement and accuracy of the system.